Garnets from xenolith in Mir kimberlite pipe: chemical composition and genesis

The problem of the lithospheric mantle structure under ancient cratons and their evolution attracts researchers in connection with the question of the diamond genesis. The petrological way is based on the mineral composition studying in xenoliths from the mantle depths. The Mirny kimberlite field belongs to the diamond-bearing kimberlite fields in the center of the Siberian craton. The authors studied a collection of mantle xenoliths from the Mir pipe (57 samples). The samples were classified as peridotites (Grt lherzolites) and pyroxenites (Grt websterites, Grt clinopyroxenites and eclogites).

Lherzolites from the Mir pipe are characterized by a high degree of alteration; olivine and orthopyroxene are replaced by serpentine in many samples (up to 50–70%). Websterite rocks are different by the presence of orthopyroxene and clinopyroxene, while clinopyroxene may contain lamellae of exsollution structures. Garnet websterites are distinguished by orange-reddish color of garnet, dark green color of pyroxene and dominanting medium-large-grained hypidiomorphic-granular textures; porphyroblastic and granoblastic textures (up to mosaic) are also observed. In garnet clinopyroxenites rutile is usually present in the form of thin (5–20 µm) needles in garnet and clinopyroxene. Eclogites are characterized by orangish or pinkish garnet color and granoblastic structure.

Garnets from lherzolites and websterites are also characterized by a relatively high Mg# content (75–83) and low TiO₂ contents (up to 0.2 wt %). It belongs to the lherzolite paragenesis by content CaO (3.68 - 5.35 wt.%) and Cr₂O₃ (0.07-3.7 wt.%). Eclogites are characterized by high-calcium (3.78 - 9.46 wt.%) and high-iron (7.77 - 17.20 wt.%) composition of garnet getting into the ​​wehrlite paragenesis area. None of the garnet studied compositions belongs to the high-chromium dunite - harzburgite paragenesis. Also garnets from the lithospheric mantle under the Mirny kimberlite field are characterized by a low-Ti garnet composition (up to 0.7 wt.%). Thus, the lithospheric mantle under the Mirny kimberlite field differs from the lithospheric mantle under other diamondiferous fields (for example, Udachnaya kimberlite pipe). The Mirny mantle xenoliths are characterized by the pyroxenites widespread development (up to 50%), the low-Ti composition and deformed lherzolites absence. These features indicate the minimal silicate metasomatic alteration in the lithospheric mantle under the Mirny field (in contrast to the center of the Siberian craton). The isotopic oxygen composition in garnet and clinopyroxene was also determined. The δ¹⁸O value varies in Cpx from 5.7-5.8‰ in clinopyroxenites and 6.1-6.1‰ in eclogites. On the whole, minerals from pyroxenites demonstrate δ¹⁸O values exceeding mantle values, which suggests a wide development of melting processes in the lithospheric mantle in the south of the Siberian craton Craton and the formation of megacrystalline pyroxene cumulates. In some cases, metamorphic recrystallization leads to oxygen isotope equilibrium between garnet and clinopyroxene. For minerals from eclogites higher values ​​of δ¹⁸O are noted, which may indicate the origin of eclogites from subducted oceanic crust, the presence of a subduction component in the process of formation of the lithospheric mantle.

The research was supported by Russian Science Foundation grant №20-77-00074.